1. Field of the Invention
The present invention relates to a technical field in which an output signal level of a frequency synthesizer, for example, is automatically adjusted by a variable attenuator.
2. Description of the Related Art
Generally, in a high-frequency apparatus, in order to make a signal level (amplitude value) of high-frequency signal constant, an APC (Auto Power Control) is performed in which a variable attenuator is provided in a high-frequency signal path and a control voltage of the variable attenuator is controlled based on a signal level obtained by a detector on an output side (at a subsequent stage) of the variable attenuator.
FIG. 10 is a circuit of a frequency synthesizer in which an APC function is incorporated. 1 denotes a PLL integrated circuit unit (PLL-IC) in which PLL (Phase Locked Loop) circuits are integrated. The PLL integrated circuit unit 1 operates to synchronize, based on a control signal from a control unit 3, a reference clock from a reference clock generating unit 11 and a frequency signal output from a voltage controlled oscillator 2 to make the high-frequency signal have a set frequency set by the control unit 3, and explanation regarding a concrete example thereof will be omitted here since it is overlapped with explanation of later-described embodiments.
A phase difference signal, for example, obtained by the PLL integrated circuit unit 1 is integrated by a loop filter 22, and a value of the integrated signal is input into the voltage controlled oscillator 2. At a subsequent stage of the voltage controlled oscillator 2, an amplifier 22, a variable attenuator 4, an amplifier 41 and a band-pass filter 42 are provided, and accordingly, the frequency signal is amplified to a predetermined signal level (amplitude value) to be an output of the frequency synthesizer. In FIG. 10, a part in which the frequency signal is set to have the predetermined frequency is indicated as an oscillation stage 100, and further, a part in which the frequency signal is amplified to the predetermined level is indicated as a variable amplification stage 200.
The output of the frequency synthesizer has to constantly keep a desired level as a reference signal of frequency, so that there is a need to constantly comply with an ambient temperature, aging of amplifier and the like.
A function to deal with the requirement is the APC function. Specifically, the control unit 3 grasps a current signal level of frequency signal based on a detection voltage (voltage detection value) detected by the detector 5, and when the signal level is deviated from a predetermined signal level, namely, when the detection voltage is deviated from a set voltage, the control unit 3 changes a control voltage of the variable attenuator 4 via a D/A (digital analog) converter 6 to adjust an attenuation amount of the frequency signal. When the attenuation amount of the variable attenuator 4 is changed, the detection voltage of the detector 5 is also changed, and the control unit 3 judges whether or not it changes the control voltage based on the detection voltage. As above, the signal level of frequency signal is set to be constant by a loop of the detector 5, the control unit 3, the D/A converter 6, the variable attenuator 4 and the detector 5.
A characteristic of the detection voltage with respect to the signal level (output level) is monotone increasing as shown in FIG. 11(a) or monotone decreasing as shown in FIG. 11(b), and when a desired output level is P1, the control unit 3 adjusts the attenuation amount of the variable attenuator 4 so that the detection voltage becomes V1. When the characteristic is monotone increasing shown in FIG. 11(a), if the detection voltage is higher than V1, the control unit 3 increases the control voltage of the variable attenuator 4 so that the output level is lowered, namely, the attenuation amount is increased, and if the detection voltage is lower than V1, the control unit 3 decreases the control voltage. Note that when the characteristic is monotone decreasing shown in FIG. 11(b), the changing operation of increasing/decreasing the control voltage by the control unit 3 is conducted in an opposite manner to the case of monotone increasing.
However, the aforementioned frequency synthesizer has a problem as follows. In the D/A converter 6, a pulse-like voltage change momentarily occurs, so that more than a little overshoot is generated on a rising edge and a falling edge. This state is schematically shown in FIG. 12. If an analog output of the D/A converter 6 is applied to the variable attenuator 4 under this state, a frequency component of the overshoot is directly superimposed on a high-frequency signal line, and in an output of the frequency synthesizer, a component corresponding to the frequency component appears as spurious at a detuned point, which results in deterioration in radio characteristics. FIG. 13 shows a result of examining a relation between a offset frequency and a signal strength (power strength) regarding the pulse-like changed portion in the D/A converter 6. A floor noise at a frequency detuned by 10 Hz or more (at a frequency of 10 Hz or more) is about equal to −80 dB, but, at a frequency of 20 kHz, the signal strength is about −30 dB, and this phenomenon is caused by an influence of overshoot.
FIG. 14 shows a spectrum waveform of an output of a frequency synthesizer, in which although a floor noise at a frequency detuned by 10 Hz or more is hidden and cannot be seen, at a position detuned by 20 kHz from a set frequency, the spurious shown in FIG. 13 directly appears. There is a case where the position of spurious differs depending on systems, and the spurious does not exert an influence on the system. However, in a frequency synthesizer used in an OFD system for terrestrial digital broadcasting with 64QAM modulation, for example, disturbance of image and sound occurs due to the spurious.
Although Patent Document 1 describes a technology in which weighting processing is performed on an output of a voltage controlled attenuator to create a control voltage and the control voltage is supplied to the voltage controlled attenuator via a D/A converter, it does not disclose a method for solving the above-described problem.    [Patent Document 1] Japanese Patent Application Laid-open No. H7-307631 (FIG. 2 and Claim 1)